1. Field of the Invention
This invention relates to electrostatics. In a particular aspect, this invention relates to the electrostatic application of material, particularly powders. In another aspect, this invention relates to an applicator for an electrostatically charged material. In another aspect this invention relates to an electrostatic gun.
2. Description of the Prior Art
It is known to electrostatically charge material such as liquids and powders and to spray the material onto a grounded article. Because of the electrostatic charge, the material will tend to cling to the article and thereafter the article may be subjected to a treatment such as drying or heating to bond the material to the article. Particular instances of this technique are spray painting of articles and coating articles with thermoplastic or thermosetting synthetic plastics materials which are thereafter heat treated to form an attractive and durable coating on the article.
In one known apparatus a stream of air in which a thermoplastic or thermosetting synthetic plastic material is entrained is passed to a gun which is directed by an operator at an object to be coated. The gun has an outlet nozzle for the air and entrained material and surrounding that nozzle is a single conductor which is charged with a high voltage.
Partly due to the entrained material passing in proximity to the conductor and partly due to the conductor ionizing air adjacent to itself, the entrained material becomes charged and, because the article to be coated is grounded, is attracted thereto.
However, a number of difficulties arise out of the use of the above described apparatus.
One difficulty is that although the conductor surrounds the nozzle, the field produced by the conductor tends to originate from the region of the conductor most adjacent to the article to be coated and in other regions of the conductor a comparatively small field is produced. Thus the entrained material which passes adjacent said other regions may not pick up sufficient charge and, in consequence, the article may be unevenly coated and/or entrained material, by not being strongly attracted to the article, may not, in fact, cling to the article and may instead be dissipated into the atmosphere.
Another difficulty is that the stronger the electrostatic field the greater is the tendency for the material to coat the article in those regions closest to the conductor rather than those areas remote from the conductor and, further, the less the tendency to coat hollow and recessed regions of the article.